Hereafter, a wireless transmit/receive unit (WTRU) includes, but is not limited to, a user equipment, mobile station fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes, but is not limited to, a base station, Node B, site controller, access point or other interfacing device in a wireless environment.
In a wireless frequency division duplex (FDD) or time division duplex (TDD) telecommunication system, timing synchronization between transmitted and received signals of a base station and a WTRU is imperative for proper two way communication. Also, Doppler effect may contribute to the frequency difference if the mobile receiver is in motion. To counteract the timing difference between the base station local oscillator and the WTRU local oscillator, a simple adjustment to the WTRU receiver local oscillator can correct the error by applying an advance or delay to the sampling rate accordingly, if there is no multipath processing in the receiver. However, due to multipath signal effects, conventional receivers of wireless communication systems employ means for detection of the multipath signals and means for reconstructing the transmitted signal, such as the RAKE type receivers.
The timing for each path is estimated in two stages. First, a channel estimator is used to find the approximate locations of each path in time for a multi-path communication channel. Second, for each path, a dedicated code tracker used in correlation to each RAKE finger finds the accurate location of the path in time and tracks it continuously thereafter. Since each path has a unique time location, controlling the code timing through the local oscillator alone does not correct the timing error in a multipath channel environment.
To address the multipath problem, code trackers may use interpolators to perform digital timing synchronization instead of controlling the local oscillator. For the efficient implementation of an interpolator, a finite impulse response (FIR) interpolator may be used. There are different known approaches for FIR interpolators. The simplest approach is to use a truncated sin c function as an FIR interpolator. Another option is to use a polynomial interpolator. Also, a minimum mean square error (MMSE) interpolator can be used. Among all of these algorithms, an MMSE interpolator provides the minimum error compared to the infinite length ideal interpolator. It is to be noted that without an efficient interpolator control unit that ensures that the interpolator is centralized with respect to the main lobe of the sin c function (i.e., centralized with respect to the interpolating function), the interpolator might result in a higher number of FIR coefficients than may be necessary for a given accuracy. The drawback to excessive coefficients is that the number of interpolation computations becomes cumbersome, and at some point, a limiting factor for the implementation. This is especially compounded as the number of trackers employed increases in order to more effectively contend with multipath effects. Thus, a tradeoff exists between extending the number of RAKE finger trackers versus the amount of time diversity gain obtained from a multi-path channel.